1. Field of the Invention
This invention relates to improvements in and relating to magnetic resonance imaging (MRI) and in particular to improvements in electron spin resonance enhanced magnetic resonance imaging.
2. Description of the Related Art
Magnetic resonance imaging is a diagnostic imaging technique that is gaining widespread acceptance among physicians. It is particularly attractive as it does not involve exposing the patient to radioactivity or ionizing radiation.
An improved modification of MRI, which utilizes the phenomenon of dynamic nuclear polarization, otherwise known as the Overhauser effect, to achieve significant enhancement of the magnetic resonance (MR) signals from which the MR images are generated, has recently been suggested. In this technique, variously termed Electron Spin Resonance Enhanced Magnetic Resonance Imaging (ESREMRI), Overhauser Magnetic Resonance Imaging (OMRI) and Proton Electron Double Resonance Imaging (PEDRI), the subject is irradiated with radiation of a frequency selected to stimulate electron spin resonance transitions in a paramagnetic substance, e.g. a stable free radical, distributed within the subject. Interaction between the stimulated electron spin system and the nuclear spin system (generally water protons) responsible for emitting the free induction delay (FID) signals from which the MR image is generated results in a relative overpopulation of the excited state of the nuclear spin system and as a result an enhancement of the FID signals. ESREMRI is discussed for example in WO-A-88/10419 (Leunbach) and related techniques are discussed in U.S. Pat. No. 4,719,425 (Ettinger), WO-A-90/02343 (Leunbach) and EP-A-302742 (Lurie).
Where MRI and ESREMRI have been used for diagnostic imaging, this has generally been to provide structural, i.e. anatomical, information. These imaging techniques can however be used to image other physical, chemical or biological parameters, for example temperature, contrast agent presence or distribution, blood flow, etc., insofar as these parameters exert a measurable effect on the magnetic resonance (FID) signals. The present invention is concerned particularly with this so-called parameter imaging. One problem generally faced in parameter imaging is that the FID signal is affected by many factors besides the parameter of interest and that if these factors vary within the sample or subject under study then variations in image intensity cannot be simply correlated to the variations in the parameter of interest. At present therefore MR images are utilized mostly for the anatomical information they provide with the image intensity being used to delineate anatomical structures rather than being considered to have any quantitative significance.
In conventional MRI, FID signal intensities are dependant on the spin-lattice and spin-spin relaxation times (T.sub.1p and T.sub.2p respectively) of the imaging nuclei, on the density of the imaging nuclei (generally the proton density) and occasionally on a flow velocity. Although it is possible to some extent to distinguish between these parameters by the use of appropriate imaging sequences, one is still faced by the problem of interpreting the significance of the relaxation times which have no clear diagnostic significance unlike primary parameters such as temperature, pH, oxygen tension, viscosity, etc.